Vehicles or vessels of different kinds are provided with panes to make it possible for the crew to look out from the interior of the vehicle, to cover sights, searchlights etc. The panes, which are transparent to the useful radiation used for the purpose, which can be IR or UV radiation but preferably light visible to the eye, are usually made of glass. When IR radiation is used, they are made of e.g. germanium. If the panes are used in combat vehicles, they should provide good defensive capability. Therefore they have a considerable thickness, in many cases more than 50 mm, and are made of laminated glass, for instance a number of sheets of toughened glass with intermediate layers of some polymer.
Modern combat vehicles should have stealth properties which are obtained, for instance, by a suitable design with flat hard surfaces. The surfaces are arranged to reflect as much as possible of the enemy radiation away from the enemy transmitter/receiver. This first enemy radiation usually comprises radar beams which are used for reconnaissance and by homing devices, but can also be some other electromagnetic radiation such as UV or laser radiation. If the vehicles have uncoated glass panes letting through radar beams, these will penetrate into the interior of the vehicle. There they will be reflected once or several times before they return to the transmitter more or less randomly. If the beams are also reflected by corner reflectors, which are often present in the interior, a considerable part of the beams is reflected back, resulting in deteriorated stealth properties.
This inconvenience is prevented by the glass panes according to prior art being arranged to reflect the radar beams so that they will be directed away from the transmitter in the same way as described above for the other parts of the vehicle. The reflection is achieved by the pane comprising, for instance, a layer which reflects radar beams and which, like the pane itself, is transparent to the useful radiation used for the purpose.
If this layer is inserted in the pane a distance down from its outer face, corner reflectors are obtained in the corners of the pane between two of the adjoining edge surfaces of the pane and the reflective layer. With this layer arranged deep down in a thick pane, the back reflection from these corner reflectors will be considerable.
Corner reflectors will, of course, not be formed if the reflective layer according to prior art is arranged on the outer face of the pane or if the pane is arranged to be reflective in some other way. The reflection may act by interference or holography but is preferably arranged by means of an electrically conductive layer of e.g. metal or metal oxide. The layer has such a thickness that visibility is not prevented but still has so low resistance that effective reflection for radar beams is obtained. Examples of materials used are gold, indium oxide and tin oxide. The latter coatings are usually preferred since they do not, like gold, cause colouring of the transmitted light.
An outer face which is highly reflective for radar has the drawback that it can get very high reflectance also for a second radiation used by the enemy. This is usually also electromagnetic radiation, which like the first enemy radiation is often used for reconnaissance and homing purposes. Specifically, the second radiation is IR radiation which as a rule is used passively and for which the pane surface, with a metal or metal oxide coating, will have very high reflectance. Since the glass material that is usually to be found in a pane does not let IR radiation through from the interior of the vehicle, the radiation from the pane, which can be perceived by an IR instrument, depends on the natural radiation of the pane as well as reflected radiation. The natural radiation depends on temperature and emittance of the body surface. In general, emittance is very low when reflectance is very high like in this case.
The other parts of the vehicle have much higher emittance than the radar-reflecting pane, usually 0.8-0.9 instead of 0.2 or less for the pane. The temperature of the other parts is usually approximately the same as that of the pane, since both of them are largely determined by the temperature of their common inner spaces. These other parts will therefore emit a radiation intensity which thus is 4-5 times greater than that of the pane since the characteristic temperatures are the same and the intensity is determined by the emittances of the surfaces.
As mentioned above, the coated pane's reflectance for IR radiation is very high. The radiation of the pane therefore consists mainly of reflection of the IR radiation from the surroundings. On stealth vehicles, the sides are frequently inclined inwards upwards, which means that the panes will reflect IR radiation from the sky. Especially in cloudless nights, a very small amount of radiation comes from the sky. This small amount of radiation thus adds very little to the small amount of emitted radiation.
The total radiation from the panes of the vehicle will thus be much smaller than the radiation from the other parts of the vehicle. By signal processing, this difference can be used to produce images, show symbols for different kinds of targets, program weapons or ammunition etc. In, for instance, image production, the panes will have a change in colour, usually dark fields, on an image of the vehicle which is otherwise light. The size and location of these differently coloured fields in the image of the vehicle facilitate recognition of the vehicle. Since recognition is the basis for an enemy's choice of a successful mode of combating, involving inter alia the choice of suitable tactics and weapons, the existence of these differently coloured fields will be highly detrimental to the vehicle in a combat situation. The mode of combating is different if the target is a heavily armoured tank or a lightly armoured tracked vehicle.